1. Field of the Invention
This invention relates generally to a system for printing a multi-toned image in electrostatographic printing apparatus, and more particularly concerns an apparatus for pre-treating developed toner on the photoreceptor before transferring it to an intermediate belt.
2. Description of Related Art
Generally, process electrostatic copying exposes an image of an original document onto a substantially uniformly charged photoreceptive member. The photoreceptor has a photoconductive layer. Exposing the charged photoreceptive member with the image discharges areas of the photoconductive layer corresponding to non-image areas of the original document, while maintaining the charge in image areas. Thus, a latent electrostatographic image of the original document is created on the photoconductive layer of the photoreceptive member. Charged developing material is subsequently deposited on the photoreceptive member. The developer material is either a liquid material or a powder material. The charged image areas on the photoconductive member attract the developer material to convert the latent electrostatic image into a visible image. The developer material is then transferred from the photoreceptive member, either directly or after an intermediate transfer step, to a copy sheet or other support substrate to create an image which is permanently affixed to the copy sheet. In a final step, any residual developer material is removed from the photoconductive surface of the photoreceptive member to prepare the photoreceptive member for a next imaging cycle.
This electrostatographic copying process is well known and is commonly used for light lens copying of an original document. Analogous processes also exist in other statographic printing applications such as, for example, ionographic printing and reproduction, where the charge is deposited on a charge retentive surface in response to electronically generated or stored images.
In multi-color electrostatographic printing, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are created. Each single-color latent electrostatic image is developed with a correspondingly colored toner. This process is repeated for a plurality of cycles. Each single-color toned image is superimposed over the previously transferred single-color toned image(s) when transferred to a copy sheet. This creates a multilayered toned image on the copy sheet. Thereafter, the multilayered toned image is permanently fixed to the copy sheet, creating a full-color copy.
In tandem color printing, to which the present invention relates, four imaging drum systems are generally used. Each imaging drum system separately charges the respective photoconductive drum, forms a latent electrostatic image on the respective drum, develops a toned image on the respective drum and then transfers the toned image to an intermediate belt. In this manner, yellow, magenta, cyan and black toned images are separately transferred to the intermediate transfer belt.
Generally, the toned images are separately transferred to the belt and superimposed on top of each other to form a four-layered toned image on the intermediate belt. When properly superimposed, these four toned images are capable of producing a wide variety of colors. Therefore, it is important to properly align and register the toned images on the belt. Each tone layer transferred to the intermediate belt is subjected to numerous electrostatic fields along the intermediate belt. Because of the electrostatic fields, the toned layers lose some of their charge, thereby decreasing the efficiency of the subsequent transfer to the copy sheet. It is therefore important to charge each toned layer to a sufficient level to enable efficient transfer to the copy sheet.
Additionally, in tandem color printing, the toner often splatters in pre-transfer zones of subsequent imaging systems. This occurs because the transferring device of each imaging system sometimes extends the transfer electrostatic field into the pre-transfer zone. A strongly charged toned image on the intermediate belt advantageously avoids toner splatter caused by any subsequent imaging systems.